This invention relates to the process of manufacture of molded plastic articles and apparatus to accomplish the process and more particularly to improvements in the process and apparatus for molding or forming under heat and pressure materials containing plastic substances which require a relatively high molding force and in which the molding material must remain under compression and heat for a predetermined cure or hardening time.
The molding of articles from materials particularly molding relatively large articles of a hardenable plastic material which may also include glass fiber reinforcement is customarily accomplished by means of heated dies in a press, usually hydraulically operated. According to conventional practice the charge of plastic material is placed in the lower die member of the press after which the upper ram of the press is caused to descend to bring the upper and lower die members together and subject the charge to heat and pressure. In accordance with some plastic molding practices, it was necessary to maintain the press closed for a substantial interval of time in order to bring the plastic charge up to temperature and thereafter to hold the material at the desired pressure and temperature for sufficient time to carry forward and complete the curing process thereof. During this period of time required to perform the molding operation with the press closed, not only is the press completely idle but also the press operator and the other workmen engaged in serving the press. In conventional methods, when taking into consideration the high cost of hydraulic presses used for these purposes, the cost of maintenance thereof, the long periods of idleness of the presses and the operators while waiting for the plastic materials to progress through the reaction stages, and the cost of plant space occupied by these presses, it will be apparent that production costs of the plastic articles are greaterly enhanced on account of these conditions.
To improve on the period of time required for the press to hold the dies in the closed position for completely curing the plastic material and making articles there has been developed a process and apparatus for using a single molding press with a plurality of individual latching molds which can be sequentially fed into and removed from the molding press. That is, a molding system utilizing a continuous process in which a plurality of latchable molds are sequentially loaded by the press in their unlatched condition, put under molding pressure, latched, and then conveyed away from the press for further curing period and then at the completion of curing the material the latched mold is unlatched and opened, the article removed, and the latchable mold returned for subsequent use again by introduction into the molding press. Such molding process and apparatus is described in U.S. Pat. Nos. 2,485,452 and 3,809,739. In U.S. Pat. No. 2,485,452 the mold units are fed in succession into and out of the press and each unit remains in the press for an interval of time during which the prliminary pressing and heating of the plastic material is carried out. Also during this interval of time the mold unit is in the press the unit may be relieved from the pressure of the press ram so as to permit breathing or escape of gases after which final pressure is applied by the press ram so as to fully close the mold and enable it to be clamped tightly in closed position for withdrawal from the press. After removing the mold unit from the press it is maintained locked until final reaction of the plastic material, and then proceeds to the final stage whereupon the mold unit may be unlocked and the molded plastic article removed. It will be noted that in U.S. Pat. No. 2,485,452 the venting of gases takes place while the mold unit is still in the press, thus tying up the press during that venting period of time. With regard to patent 3,809,739 there is described therein a molding system that utilizes a continuous process in which the latchable mold assemblies are sequentially loaded by the press, latched, put under additional molding press by an expandable device such as a pneumatic bag, and are then conveyed away from the press, during the curing period, to an unloading station where, after unloading, they are returned to the press for subsequent use. This molding process and equipment requires substantial additional apparatus in order to have a fluid supply and vent connected to the interior of the pneumatic bag and to a fluid pressure supply and controls which are operated to supply fluid under pressure to the bag and to vent the bag for deflation thereof.
By this invention, there is provided an apparatus that allows for high speed molding of articles without tying up a molding press for a substantial period of time by utilizing a mold assembly that will have the pressure applied by a press and the assembly locked and removed from the press for further curing under correct pressure, temperature and time and which compensates for exhausting gases and shrinkage of the material during the initial curing process without the need of a separate pneumatic bag with its attendant equipment necessary to pressurize the bag and then vent the bag after completion of the curing process.